X1800 Booth Output

Hi, tried connecting to booth output but am getting no audio out.

Could it be because I am using the mixer as an external mixer in Traktor ?

So doing some further investigating the booth out on this X1800 does not work connected via TRS Jack into an audio interface (SSL 2+) in monitor mode (master out balanced and unbalanced works fine with this config as does other gear however) but it does work when connected directly to a speakers RCA out via a TS Jack to RCA cable.